Numerous applications exist where the liquid lubricants are not suitable because they “drift” with respect to the lubrication point. These are in particular rolling-contact bearings and slider bearings, open gear sets, metal cables and chain drives, and more generally for applications not comprising a sealing system. For these applications, lubricating greases are used, which are solid or semi-fluid substances resulting from the dispersion of a thickener in a liquid lubricant, optionally incorporating additives which give them specific properties.
The thickeners can be organic or inorganic compounds. Among the organic thickeners used in the manufacturing of greases, the fatty acid metal salts and polycarbamides (polyureas) can in particular be mentioned.
The vast majority of the lubricating greases are prepared with thickeners of the fatty acid metal salt type. The fatty acid is dissolved in the base oil at a relatively high temperature, and then an appropriate metal hydroxide is added. After evaporating the water which forms during the reaction by boiling, cooling is carried out for a precise amount of time, in order to form the soap lattice.
Lithium, sodium, calcium, barium, titanium or aluminium hydroxides, or certain aluminium trimers, are suitable for example as metal compounds for manufacturing grease. Long-chain fatty acids, of the order of C14 to C28, mainly C18, generally originate from vegetable (castor oil for example), or animal (for example tallow) oils. They can be hydrogenated or hydroxylated. The best-known derivative is 12-hydroxystearic acid originating from ricinoleic acid.
In combination with the long-chain fatty acids, it is also possible to use short-chain acids, typically comprising between 6 and 12 carbon atoms, such as for example azelaic acid, benzoic acid. Other, in particular inorganic, thickeners such as, for example, bentonite and silica gel can be used. For applications where the grease is located in an unconfined enclosure (for example open gear sets in cement works etc.), the greases thickened with metal soaps, and in particular with simple or complex metal soaps of aluminium are greatly superior to the other greases. The greases thickened with polyureas do not have sufficient mechanical stability, in particular due to their thixotropic nature, which leads to their becoming destructured under mechanical stresses. The inorganic thickeners also present problems of mechanical strength and water resistance.
The greases thickened with a calcium sulphonate complex soap (or calcium sulphonate complex greases) have been known and used for many years, as they have numerous properties such as extreme-pressure and anti-wear properties, mechanical resistance, corrosion resistance, water resistance and thermal stability, in particular at high temperatures. This type of grease is obtained from the conversion of an overbased calcium sulphonate in the presence in particular of at least one base oil, at least two different acids, one of which is a fatty acid and at least one base (cf. Gareth Fish et al, “Calcium Sulphonate Grease Formulation”, 2012). They find their application in numerous industrial fields, in particular automobiles, the steel industry, mining operations or also paper manufacturing.
Several processes for the preparation of calcium sulphonate complex greases have already been described or implemented. Single-phase processes for the preparation of calcium sulphonate complex greases have in particular been described; the objective being to reduce the preparation time while maintaining or even improving the yield. By single-phase preparation process, is meant more particularly a process for the preparation of a calcium sulphonate complex grease comprising a single continuous rise in temperature and a single fall in temperature.
U.S. Pat. No. 5,338,467 describes a process for the preparation of a calcium sulphonate complex grease, the particles of calcium carbonate being in the form of calcite, said process being able to be implemented in a single phase and being able to include pressurizing the mixture constituting the grease. However, the examples of the process cited in this document all describe the presence of boric acid. The same is true for U.S. Patent Publication No. 2013/220704.
U.S. Pat. No. 4,560,489 describes a process for the preparation of a calcium sulphonate complex grease being able to be implemented in one phase and can include pressurizing the mixture constituting the grease. Moreover, this document describes that this process can be implemented in the absence of boric acid. However, this pressurization is carried out by the introduction of CO2 into the reactor comprising the mixture. Moreover, the examples described in this document show the importance of the presence of boric acid on the thermal stability properties of the grease obtained at the end of the process. In fact, in the absence of boric acid, the thermal stability of the grease obtained at the end of the process is very low, whereas this stability improves with the increase in the boric acid content.
Document CN 102703185 describes a single-phase process for the preparation of a calcium sulphonate complex grease and the mixing of different components in a reactor under pressure. However, the process described in this document includes the presence of boric acid. Moreover, the pressurization of the mixture in the reactor is obtained by the addition of CO2. Moreover, the process described in this document requires the presence of a co-solvent of the methanol or ethanol type, these co-solvents being able to give off volatile organic compounds (VOCs). Now, it is known that these compounds can represent a potential danger to human health.
It would therefore be desirable to have a process available for the preparation of a calcium sulphonate complex grease, which can be implemented both in a single phase and in the absence of boric acid. In fact, boric acid is a product classed as CMR (carcinogenic, mutagenic or toxic to reproduction) and therefore represents a potential danger to human health. It would also be desirable to have available a single-phase process for the preparation of a calcium sulphonate complex grease making it possible to significantly reduce the preparation time and maintain or even increase the yield, while retaining or even improving the properties of the grease. It would also be desirable to have available a single-phase process for the preparation of a calcium sulphonate complex grease, comprising at least one step of pressurizing the mixture constituting the grease, this pressurization not requiring the addition of gas, and in particular of carbon dioxide. It would also be desirable to have available a single-phase process for the preparation of a calcium sulphonate complex grease, not requiring the addition of solvents or co-solvents which give off VOCs.
An objective of the present invention is to provide a process overcoming all or some of the aforementioned drawbacks. Another objective of the invention is to provide a simple process which can be easily implemented. Another objective of the invention is to provide a calcium sulphonate complex grease the thermal stability of which is improved.